Response of Cucumber Plants to Low Doses of Different Synthetic Iron Chelates in Hydroponics

The factors that control the use of iron (Fe) provided by iron chelates in strategy I plants are not well known. In this paper, the effectiveness of low concentrations of a series of pure Fe chelates to supply Fe to cucumber plants in hydroponics was studied. The Fe Chelate Reductase (FCR) of the roots was measured using Fe- ethylene diamine tetraacetic acid (EDTA) as substrate. Despite the differences found in SPAD and biometric indexes among the treatments, FCR and Fe in xylem sap were only significantly larger for the Fe- Ethylene diamine di-(o-hydroxy-p-methylphenyl) acetic acid (EDDHMA) treatment. The trend in nutritional indexes was the opposite to the trend in the stability of the chelates, except for Fe-EDTA that gave the poorest results. A mechanism describing the uptake process, considering the re-oxidation of the Fe (II) reduced by the FCR and the formation of the Fe (II) complex is proposed.     

Partial replacement of Fe(o,o‐EDDHA) by humic substances for Fe nutrition and fruit quality of citrus

The most widely used Iron (Fe) fertilizer in calcareous soils is the synthetic chelate Fe(o,o‐EDDHA). However, humic substances are occasionally combined with Fe chelates in drip irrigation systems in order to lower costs. We investigated the effect of various mixtures of Fe(o,o‐EDDHA) and a commercially available humic substance on Fe availability in a calcareous soil from Murcia, Spain (in vitro experiment) and on leaf Fe content and fruit‐quality attributes of Citrus macrophylla (field experiment). In the in vitro experiment, a calcareous soil was incubated for 15 d with solutions of sole Fe(o,o‐EDDHA) and humic substance and of a mixture of humic substance and Fe(o,o‐EDDHA) to determine the dynamics of available Fe. While the mixture did not significantly increase the available soil Fe, it did decrease the rate of Fe retention in the surface soil compared to sole Fe(o,o‐EDDHA). In the field experiment, the substitution in the application solution of 67% of Fe(o,o‐EDDHA) by commercial humic substance increased leaf P in lemon trees from 0.19% with sole Fe(o,o‐EDDHA) to 0.30% and leaf Fe from 94 mg kg^–1 to 115 mg kg^–1. Some quality parameters like vitamin C content and peel thickness were also improved with a partial substitution of Fe(o,o‐EDDHA) by humic substances. We conclude that a partial substitution of commercial Fe chelates by humic substance can improve crop Fe uptake and may thus be economically attractive. The underlying physiological mechanisms and ecological implications require further studies.     

Iron Chlorosis in Gerber as Related to Properties of Various Types of Compost used as Growing Media

Abstract

Nutrient deficiencies in crop plants may be influenced by a number of properties of the growing media. Some peat‐substitute substrates can promote iron (Fe) chlorosis in sensitive plants, which has traditionally been ascribed to the elevated pH of growing media. To identify the origin of this problem in various types of composted organic residues used as growing media and possible corrections, a complete randomized experiment on gerber (Gerbera jamesonii Adlam) as an Fe‐chlorosis sensitive crop involving three factors (growing medium, medium acidification, and the medium treatment with Fe) was performed.

Although the Fe content in plants decreased with increasing pH in the growing medium, the chlorophyll content as measured using a chlorophyll meter (Minolta Soil Plant Analysis Development, SPAD) was not significantly related to pH. The SPAD readings and Fe concentrations in plants, dry matter, and flower production were not significantly related to diethylenetriaminepentaacetic acid (DTPA)–extractable Fe in the growing media. The addition of Fe‐chelate significantly increased yield (P<0.01), and SPAD at 65 and 96 days after planting (P<0.001 and P<0.01, respectively). However, the effect of the acid treatment was, different depending on the growing media. When the acidification promoted a positive effect on SPAD readings, this was nonsignificantly different than that obtained with the application of Fe‐chelate. The estimated amount of available Fe in the growing media was not relevant, which explains the incidence of chlorosis as physiological factors related to pH.     

Iron fertilizers applied to calcareous soil on the growth of peanut in a pot experiment

A greenhouse pot experiment was conducted with peanuts (Arachis hypogaea L., Fabceae) to evaluate iron compound fertilizers for improving within-plant iron content and correcting chlorosis caused by iron deficiency. Peanuts were planted in containers with calcareous soil fertilized with three different granular iron nitrogen, phosphorus and potassium (NPK) fertilizers (ferrous sulphate (FeSO₄)–NPK, Fe–ethylendiamine di (o-hydroxyphenylacetic) (EDDHA)–NPK and Fe–citrate–NPK). Iron nutrition, plant biomass, seed yield and quality of peanuts were significantly affected by the application of Fe–citrate–NPK and Fe–EDDHA–NPK to the soil. Iron concentrations in tissues were significantly greater for plants grown with Fe–citrate–NPK and Fe–EDDHA–NPK. The active iron concentration in the youngest leaves of peanuts was linearly related to the leaf chlorophyll (via soil and plant analyzer development measurements) recorded 50 and 80 days after planting. However, no significant differences between Fe–citrate–NPK and Fe–EDDHA–NPK were observed. Despite the large amount of total iron bound and dry matter, FeSO₄–NPK was less effective than Fe–citrate–NPK and Fe–EDDHA–NPK to improve iron uptake. The results showed that application of Fe–citrate–NPK was as effective as application of Fe–EDDHA–NPK in remediating leaf iron chlorosis in peanut pot-grown in calcareous soil. The study suggested that Fe–citrate–NPK should be considered as a potential tool for correcting peanut iron deficiency in calcareous soil.     

铁还原和硫酸盐还原对红壤中二苯砷酸释放与硫化的影响

二苯砷酸（diphenylarsinic acid,DPAA）是含砷化学武器在环境中的主要降解产物之一,研究铁还原和硫酸盐还原对土壤中DPAA释放与硫化的影响对于深入认识DPAA的环境地球化学行为至关重要。考察了在淹水和添加不同浓度乳酸钠（C）和硫酸钠（S）培养时花生地和林地红壤中DPAA的释放与硫化情况,并分析铁还原和硫酸盐还原的影响。结果显示：（1）对花生地而言,与不添加硫酸钠和乳酸钠（S-C-）以及仅添加硫酸钠（426 μg?g-1,以S计,S+C-）的处理相比,同时添加硫酸钠和乳酸钠（1 300 μg?g-1（以C计,下同）,S+C+;2 170 μg?g-1,S+C++）的处理中DPAA释放明显增强,这与乳酸钠促进了铁还原（>45%）从而有利于DPAA的释放有关;（2）在花生地的S+C+处理中硫酸盐还原最为显著（13周时溶解态硫化物浓度为11.28 mg?L-1）,DPAA的去除率（59.6%）最高且主要发生硫化,而在S+C++处理中铁还原程度的增加和硫酸盐还原程度的降低不利于DPAA的硫化;（3）与花生地不同,无论是否添加乳酸钠和硫酸盐,林地中未见DPAA的释放与硫化现象,这与其铁还原程度较低（<25%）和硫酸盐未发生还原（未检测到溶解态硫化物）有关;（4）与林地相比,花生地中梭菌属、芽孢杆菌属和脱硫芽孢杆菌属的丰度明显增加,这很可能分别促进了铁还原和硫酸盐还原,最终对DPAA释放和硫化产生了更显著的影响。总体而言,与铁还原促进DPAA的释放和进一步的硫化相比,硫酸盐的还原程度以及溶液相中硫化物的浓度很可能是控制淹水红壤中DPAA硫化的一个更重要的因素。研究结果将为深入认识DPAA的环境行为和发展DPAA污染红壤的生物刺激修复技术提供理论基础。     

Siderophore-Producing Rhizobacteria as a Promising Tool for Empowering Plants to Cope with Iron Limitation in Saline Soils: A Review

Iron (Fe) bioavailability to plants is reduced in saline soils; however, the exact mechanisms underlying this effect are not yet completely understood. Siderophore-expressing rhizobacteria may represent a promising alternative to chemical fertilizers by simultaneously tackling salt-stress effects and Fe limitation in saline soils. In addition to draught, plants growing in arid soils face two other major challenges:high salinity and Fe deficiency. Salinity attenuates growth, affects plant physiology, and causes nutrient imbalance, which is, in fact, one of the major consequences of saline stress. Iron is a micronutrient essential for plant development, and it is required by several metalloenzymes involved in photosynthesis and respiration. Iron deficiency is associated with chlorosis and low crop productivity. The role of microbial siderophores in Fe supply to plants and the effect of plant growth-promoting rhizobacteria (PGPR) on the mitigation of saline stress in crop culture are well documented. However, the dual effect of siderophore-producing PGPR, both on salt stress and Fe limitation, is still poorly explored. This review provides a critical overview of the combined effects of Fe limitation and soil salinization as challenges to modern agriculture and intends to summarize some indirect evidence that argues in favour of siderophore-producing PGPR as biofertilization agents in salinized soils. Recent developments and future perspectives on the use of PGPR are discussed as clues to sustainable agricultural practices in the context of present and future climate change scenarios.     

Transformations and Availability of Iron to Wheat as Influenced by Phosphorus and Manganese Fertilization in a Typic Haplustept Soil

An investigation was conducted using Typic Haplustept, sandy loam soil, to investigate the interactive effects of phosphorus (P) and manganese (Mn) fertilization on native iron (Fe) pools in soil and their availability to wheat (cv. PBW-343) crop. Phosphorus fertilization moved Fe from residual mineral fraction of Fe to manganese oxides (MnOX), organic matter (OM), amorphous (AMPOX), and crystalline (CRYOX) Fe and Al oxide fractions. However, Mn application decreased specifically adsorbed (SAD)–Fe and CRYOX–Fe but increased OM–Fe and mineral fraction of Fe. Available Fe in soil decreased as Olsen P and P:Mn ratio increased in the soil. Higher Olsen P (>60 mg P kg^?1soil) reduced mean Fe uptake by shoot. P content and P:Mn ratio in soil as well as in root and shoot were inversely related to Fe concentration in both the plant parts. The role of soil Fe associated with oxides and organic matter was found most notable in Fe nutrition of wheat.     

Biochemical Responses to True and Bicarbonate-Induced Iron Deficiency in Grapevine Genotypes

ABSTRACT

Biochemical responses to direct or bicarbonate-induced iron (Fe) deficiency were compared in two Tunisian native grapevine varieties, Khamri (tolerant) and Balta4 (sensitive), and a tolerant rootstock, 140Ru. Woody cuttings of each genotype were irrigated with a nutrient solution containing one of the following: 20 μM Fe (control), 1 μM Fe (direct Fe-deficiency), or 20 μM Fe + 10 mM HCO₃ ^? (indirect bicarbonate-induced Fe-deficiency). Under direct Fe-deficient conditions, lower leaf chlorosis score and higher chlorophyll and leaf Fe contents were found in Khamri and 140Ru compared with Balta4. Moreover, indirect Fe deficiency caused similar effects on these parameters, which were more pronounced in Balta4. Both tolerant genotypes, Khamri and 140Ru, showed higher roots-acidification capacity and phenol release under the direct Fe deficiency compared with the bicarbonate-induced condition. In the sensitive variety, Balta4, no significant changes were found between the control and Fe-deficient plants. Root Fe(III)-reductase activity was strongly stimulated by both types of Fe deficiency in Khamri and 140Ru, and displayed no significant changes in Balta4. In the three genotypes, root and leaf activities of two Fe-containing enzymes, catalase and guaiacol peroxidase, were significantly affected under Fe deficiency (either direct or induced), though to a greater extent in the sensitive variety, Balta4. The latter also displayed higher leaf malonyldialdehyde (MDA) content, traducing an important membrane lipid peroxidation.     

腐殖酸对外源砷在土壤中形态转化和有效性的影响

通过室内培养的方法,探讨腐殖酸(HAs)的两种活性组分——富里酸(FA)和胡敏酸(HA)及其不同比例(HA/FA)对土壤As形态转化及有效性的影响。结果表明,HAs的两种组分对土壤As的形态均具有显著影响,其影响程度和方向与其浓度、比例及外源As含量有关。FA和添加量≤1%C的HA能增加土壤交换态As(Ex-As)的含量,添加量≥3%C的HA能降低土壤Ex-As含量;两种组分都能降低土壤铝型As(Al-As)和铁型As(Fe-As)含量,增加残渣态As(Res-As)含量,FA比HA的作用效果更强;在等量(1%C)HAs条件下,HA/FA≥5/5时HAs能降低Ex-As含量,≤3/7时则能增加Ex-As的含量。HAs对土壤有效As的影响是其对As形态分配综合作用的结果,对土壤有效As贡献最大的是Ex-As和Ca-As;HA和添加量较高(≥5%C)的FA能够降低外源As在土壤中的有效性,而添加量较低(≤3%C)的FA能在一定程度上增加土壤As的有效性。因此,掌握有机物料合理的用量和活性组分比例是其作为土壤As活性调控剂的前提。     

Physical, chemical and microbiological properties of an Andisol as related to land use and tillage practice

The effects of land use and management practice on soil physical, chemical and microbiological properties may provide essential information for assessing sustainability and environmental impact. This study compared the effects of 41 years of no-tillage (NT) with continuous apple orchard, with those of conventional tillage (CT) with wheat–soybean rotation and another of puddling (PD) with continuous rice on the characteristics of a pumice Andisol in a temperate region of northern Japan. Higher values for bulk density, penetration resistance, pH, C/N ratio, exchangeable Na (X-Na), Fe, and Mn were observed for PD than NT and CT. On the other hand, organic matter, EC, N, exchangeable K (X-K), exchangeable Ca (X-Ca) and Cu were significantly higher for NT than CT and PD. Highest content of Zn was found in CT compared to other practices. The three-phase composition at pF 2.0 was significantly affected by land use and tillage practices. The solid phase and liquid phase were greater under PD than under NT and CT, while air phase was greater under CT than under NT and PD. Significantly higher values for saturated hydraulic conductivity was found in CT than NT and PD. Total phospholipid fatty acid (PLFA) and PLFA for bacteria, aerobes and cyanobacteria were remarkably higher in NT than CT and PD, regardless of depth. On the other hand, PLFA for methane-oxidizing bacteria, sulfate-reducing bacteria and mycorrhizae were significantly higher in CT than NT and PD. PLFA for fungi was significantly higher in surface (0–10 cm) soils than subsurface (10–20 cm) soils regardless of treatments. Highest bacterial and fungal diversity evaluated by DNA band number in DGGE analysis based on PCR amplification of 16S rDNA and 18S rDNA fragments, respectively, were observed in surface soil of PD. The result suggests a linkage between microbial community and tillage practices in temperate Andisol. This study also justifies the need of measuring soil characteristics based on soil microbial communities.     

外源胡敏酸对土壤有机质组分变化的影响及其与茶树铅有效性的关系

盆栽试验分析了胡敏酸输入土壤6个月后茶树根际和非根际土壤有机质SOM及其组分,包括水溶性物质WSS、富啡酸FA、胡敏酸HA以及土壤微生物生物量SMB的变化,并对不同土壤有机质组分碳含量和茶树不同组织铅含量作了相关性分析。外源胡敏酸设置3个浓度水平（0、300、600 mg kg-1）,铅设置2个浓度水平（0、300 mg kg-1）,试验共有6个处理。茶苗置于温室培养,6个月后分新叶、老叶、新茎、老茎、根收获。结果表明：根际并非总是富含有机质,未加铅处理根际SOM含量低于非根际;而加铅处理根际SOM含量高于非根际。HA加入土壤能够显著增加根际和非根际SOM和SMB的含量,但是加铅处理和不加铅处理增加幅度不同。高浓度HA显著增加了根际FA的含量,可能是HA部分裂解所致。非根际有机质各组分与茶树铅含量无显著相关关系;根际SOM、HA和茶树不同组织铅含量没有显著相关关系。根际WSS、FA与茶树嫩叶、老叶、嫩茎、老茎以及根部铅含量均存在显著或极显著正相关关系;而根际SMB与茶树各个组织铅含量存在显著负相关关系。总之,胡敏酸施入土壤改变了土壤有机质及其各个组分的含量,而根际某些土壤有机质组分的改变又显著影响了茶树不同组织铅的吸收和分布。     

根施5-氨基乙酰丙酸对亚适宜温光下黄瓜幼苗生长的影响

为缓解亚适宜温光环境对黄瓜幼苗生长产生的不利影响,以中农26号为试验材料,研究亚适宜温光下根施不同浓度5-氨基乙酰丙酸(ALA)对黄瓜幼苗生长和生理特性的影响。结果表明,亚适宜温光下,根施5 mg·L^-1 ALA可以显著提高黄瓜幼苗的全株干重、壮苗指数和根系活力,与对照相比,黄瓜幼苗全株干重和壮苗指数分别提高了32.5%和47.1%,根系活力提高了95.0%。根施5 mg·L^-1 ALA能够显著增加幼苗叶片叶绿素和ALA含量,与对照相比,总叶绿素和ALA的含量分别提高了49.5%和55.7%。根施5 mg·L^-1 ALA黄瓜幼苗的净光合速率(Pn)和气孔导度(Gs)分别较对照提高了22.9%和42.2%。根施5 mg·L^-1ALA后,抗氧化酶活性显著提高,叶片和根系的丙二醛(MDA)含量分别比对照降低了35.3%和18.1%。同时,根施适宜浓度的ALA对矿质元素的吸收和运输产生有一定影响,但对叶绿素合成过程中相关基因的表达无明显调控作用。本研究为缓解黄瓜幼苗亚适宜温光伤害提供了新方法,也为ALA在设施农业上的应用提供了技术支持。     

Leaf Responses to Reduced Iron Availability in Two Tomato Genotypes: T3238FER (Iron Efficient) and T3238fer (Iron Inefficient)

Abstract

The present work is aimed at evaluating some effects induced by different levels of iron availability in the growth medium for two different tomato (Lycopersicon esculentum Mill.) genotypes, the T3238fer (Tfer), unable to activate mechanisms for iron mobilization and uptake known as “strategy I,” and its correspondent wild‐type T3238FER (TFER). By using different iron concentration in the growth solution, the most suitable iron level to induce phenotypic differences between the two genotypes without being lethal for the mutant was found to be 40 µM Fe‐Na‐EDTA. The analyses were carried out also on plants grown with 80 µM Fe‐Na‐EDTA, an iron concentration at which the two genotypes showed no phenotypic differences. A significant decrease in total leaf iron and chlorophyll content was detected in both genotypes following reduction of iron concentration in the nutrient solution, and was particularly evident in Tfer40, which showed symptoms of chlorosis. The photo‐electron transport rate of the whole chain was significantly affected by growth conditions as well as by genotype, the lowest activity being detected in Tfer40 plants. Chlorophyll a fluorescence analysis revealed an increase in non‐photochemical quenching (q _NP) of Tfer plants grown at both iron concentrations, indicating the activation of photoprotective mechanisms, which, however, were not sufficient to prevent photoinhibition when plants were grown at 40 µM iron, as indicated by significant reduction in PSII photochemistry (F _v/F _m) and photochemical quenching (q _P). The actual quantum yield of PSII (Φ_PSII) and the intrinsic PSII efficiency (Φ_EXC) showed the same behavior of q _P and F _v/F _m ratio. A significant effect of mutation and iron supply on all the pigments was detected, and was particularly evident in the mutant grown at 40 µM iron. A different behavior was shown by the three pigments involved in the xantophyll cycle, violaxanthin being less affected than chlorophylls and the other carotenoids, and zeaxanthin even increasing, due to the xanthophyll cycle activation. In conclusion, the interaction between iron deprivation and fer mutation induced functional alterations to the photosynthetic apparatus. Anyway, as far as concerns the photo‐electron transport activity, the influence of fer mutation seemed to occur independently from iron supply.     

Influences of past application rates of nitrogen and a catch crop on soil microbial communities between an intensive rotation

The aim of this study was to investigate influences of six-year past application rates of nitrogen and a catch crop, sweet corn (Zea mays L. ssp. Saccharata Sturt), on soil microbial community and diversity in a greenhouse-based intensive vegetable soil in eastern China. Soil electrical conductivity, pH, mineral nitrogen, phospholipid fatty acids (PLFA) profiles and carbon source utilization patterns under five annually past nitrogen rates (0, 348, 522, 696 and 870?kg?nitrogen?ha^?1) were evaluated after the establishment of sweet corn during 1–1.5-month fallow period over three-year tomato/cucumber/celery rotations. The past nitrogen application rates exerted significant effects on soil electrical conductivity, pH, nitrate-nitrogen, ammonium-nitrogen and carbon source utilization patterns, but not on PLFAs profiles. The sweet corn had a significant effect on soil chemical properties, total and actinobacterial PLFAs, but not on carbon source utilization patterns. Soil electrical conductivity, nitrate-nitrogen and the total PLFAs decreased whilst soil organic carbon, pH and the actinobacterial PLFAs increased after the establishment of sweet corn. Soil microbial functional diversity from carbon source utilization patterns and actinobacterial PLFAs were greatest after the establishment of sweet corn at a 60% of the conventional nitrogen rate (i.e. 522?kg?nitrogen?ha^?1). Soil electrical conductivity and ammonium-nitrogen were two key factors to determine carbon source utilization patterns, whilst soil pH was the key factor to determine PLFAs profiles. A combination of the catch crop sweet corn during summer fallow and a 60% of the conventional nitrogen rate is a sustainable pathway of utilizing greenhouse-based intensive vegetable soils in eastern China.     

Growth,P accumulation,and physiological characteristics of two ecotypes of Polygonum hydropiper as affected by excess P supply

The use of suitable plants that can accumulate excess phosphorus (P) from contaminated soil may serve as an attractive method for phytoremediation. In this study, pot experiments were conducted to investigate the effects of P incorporation on P accumulation and physiological mechanisms of Polygonum hydropiper in a mining ecotype (ME) and nonmining ecotype (NME) from a phosphorus mining and a noncontaminated agricultural area, respectively. The results demonstrate that the ME of P. hydropiper growing in soil supplied with 0, 100, 200, 400, 800, 1600 mg P (kg soil)^–1 showed a significantly higher biomass compared to the NME. Phosphorus accumulation of the ME was positively correlated with the soil P concentration. APase activity in roots of the ME significantly increased at 1600 mg P (kg soil)^–1 and phytase activity of the ME increased with increasing P supply. APase activity of the ME was more than twice that of the NME on average. A significant increase of superoxide dismutase (SOD) was observed compared with the NME at all supplied P levels. Peroxidase (POD) activity of the ME was significantly higher at 200 and 400 mg P (kg soil)^–1. No statistical differences in the catalase (CAT) activity of the ME were observed compared with the control. Activity of CAT in the NME was obviously induced after exposure to 100–800 mg P (kg soil)^–1. Malondialdehyde (MDA) concentration in leaves of the ME decreased with increasing P supply to reach a minimum at 400 mg P (kg soil)^–1. In the NME, an increase in MDA concentration compared to the control was observed at higher P levels. The APase and phytase induction and antioxidative defense allowed for the high P accumulation of the ME.     

轻壤质潮土氮肥基追比对小麦产量与品质的影响

田间试验研究轻壤质潮土氮肥基追比对冬小麦产量和品质的影响。结果表明:供试麦田氮肥施用的基追比例以4:6为宜,追肥氮在拔节期施用,可获最高小麦子粒产量。基追比不同对小麦品质也有较大影响,在基追比5～6:5～4的范围内,籽粒蛋白质含量、产量、湿面筋、沉淀值均相对较高。生育期氮素积累量对蛋白质含量与产量有显著的影响。     

适应玉米的溶磷细菌筛选及其对玉米生长的影响

从石灰性土壤中分离获得4株高效溶磷细菌X5、X6、Z4和Z8,研究其生物学特征,探索其单独及复合的溶磷促生潜能。研究发现菌株X5、X6、Z4和Z8均可以利用玉米根系分泌物作碳源生长。菌株X6和Z4均能产生吲哚乙酸(IAA)和铁载体,菌株Z8可产生IAA不产生铁载体,菌株X5可产生铁载体不产生IAA。盆栽试验结果表明,接种单一溶磷菌及4株菌复合处理均可促进玉米生长,但复合菌群的溶磷促生效果显著高于单一菌株。通过16S r RNA基因序列分析研究菌株的分类地位,初步鉴定X5、X6、Z4、Z8分别为荧光假单孢菌(Pseudomonas fluorescens)、草假单胞菌(Pseudomonas poae)、巨大芽孢杆菌(Bacillus megaterium)和枯草芽孢杆菌(Bacillus subtilis)。     

Forest floor microbial communities in relation to stand composition and timber harvesting in northern Alberta

With the growing interest in silvicultural techniques that more closely emulate natural disturbance regimes, there is a need to better understand how partial harvesting affects the soil microbial community in stands with varying ecological characteristics, e.g., tree species composition. Four and a half and 5.5 years post-harvest, we used phospholipid fatty acid (PLFA) and substrate-induced respiration (SIR) analyses to compare the microbial biomass and microbial community structure of forest floors from stands dominated by white spruce (Picea glauca; SPRUCE) or by trembling aspen (Populus tremuloides; ASPEN) and from mixed-species (MIXED) stands in northern Alberta, Canada, that had been clearcut, partial-cut with 20% retention, partial-cut with 50% retention or left uncut (controls). PLFA and SIR analyses revealed that ASPEN forest floors supported a larger microbial biomass with a very different community structure than MIXED or SPRUCE forest floors. The microbial community structure of these soils appeared to be strongly affected by the presence of white spruce and the composition of the understory vegetation. There were no effects of timber harvesting detected within or across stand types on any of the variables measured, with the exception of the PLFA 16:1ω5, which was relatively more abundant in the clearcuts and 50% retention treatments than in the uncut controls, perhaps in response to an increased forest floor pH and grass cover in the disturbed areas. The resilience to timber harvesting of the forest floors from these stands may be the result of efforts to minimize soil disturbance during harvesting and to allow vegetation to regenerate naturally. From the perspective of the forest floor microbial community, partial harvesting does not appear to have any benefit over clearcut harvesting at these boreal forest sites.     

Molecular Monitoring of SRB Community Structure and Dynamics in Batch Experiments to Examine the Applicability of in situ Precipitation of Heavy Metals for Groundwater Remediation (15 pp)

Background, Aims and Scope   Sulfate-reducing bacteria (SRB) are known for their capacity to reduce and precipitate heavy metals (HM) as metal sulfides, offering the opportunity to create an in situ reactive zone for the treatment of heavy metal-contaminated groundwater, a process called in situ metal precipitation (ISMP). The applicability of the ISMP technology first has to be investigated at a laboratory scale before going into an on site application. The evaluation and optimization of the ISMP process is facilitated when physical/chemical analysis techniques are combined with molecular tools that specifically monitor the abundance, diversity and dynamics of the indigenous sulfate reducing microbial community. In this study, batch experiments were conducted in order to investigate the feasibility of ISMP as a groundwater remediation strategy for an industrial site contaminated with elevated levels of Zn, Cd, Co and Ni. Methods   The potential of different types of carbon source/ electron donor (lactate, acetate, methanol, ethanol, Hydrogen Release Compound?, molasses) to stimulate the sulfate reduction and metal precipitation activity of the naturally present (or indigenous) SRB community was explored. In addition, the effect of amending vitamin B12 and yeast extract was evaluated. The ISMP process was monitored by combining analytical analyzes of process parameters (SO42&#61485;-concentration, heavy metal concentrations, pH, Eh) with molecular tools such as SRB subgroup and genus specific PCR, denaturing gradient gel electrophoresis (DGGE), and phylogenetic analysis of clone sequences, based on either the 16S rRNA or the dsr (dissimilatory sulfite reductase) gene. Results and Discussion   The efficiency of different carbon-sources to stimulate the ISMP process followed the order HRC&#61650; 〉 molasses 〉 methanol 〉 lactate 〉 ethanol 〉 acetate. Within 10 weeks, the highest sulfate and metal removal efficiencies ranged from 85% to 99%. Addition of yeast extract boosted the ISMP process, whereas vitamin B12 negligibly affected SRB activity. Analysis of the sulfate reducing population by SRB subgroup and genus specific PCR demonstrated that members of the genus Desulfosporosinus dominated in all batch tests, while 16S rDNA DGGE profiles additionally revealed the presence in the microbial communities of non-sulfate reducing bacteria within the family Clostridium and the &#61541;-proteobacteria. The dsrB-based DGGE profiles allowed us to assess the diversity and dynamics of the sulfate reducing community and added to a better understanding of the effects of different batch conditions on the ISMP process. Remarkably, all dsrB sequences affiliated with the dsrB gene sequence cluster found in Desulfotomaculum, which received their xenologous dsrB gene from the &#61540;-proteobacteria. Conclusions   The batch experiments, which aimed at stimulating the activities of the indigenous SRB communities, demonstrated that these communities were present and that their activities could be used to obtain efficient in situ precipitation of the contaminating heavy metals. This opens the possibility to test this concept in the future as an on site demonstration as part of the groundwater strategy for the heavy metal contaminated site. Although batch setups are suitable for preliminary feasibility studies for ISMP, they do not reflect the in situ situation where sulfate and heavy metal and metalloid polluted groundwater are supplied continuously. A sulfate reducing strain JG32A was isolated from whose 16S rRNA gene affiliated with the genus Desulfosporosinus, while its dsrB gene sequence clustered with Desulfotomaculum dsrB gene sequences, which received their xenologous dsr genes from &#61540;-proteobacteria. Therefore we hypothesize that the batch experiments enrich members of the Desulfosporosinus genus that possess a non-orthologous dsrB gene. Recommendation and Perspective   The next step towards an on site pilot test for ISMP will be the setup of a series of column experiments, with process conditions that are selected based on the above mentioned results. This will allow to define optimal ISMP process conditions and to test its long-term efficacy and sustainability before going into an on site bioremediation application. By applying the described molecular tools together with physical-chemical analyzes, it can be investigated whether the same SRB community is enriched and which type of C-source is most effective in promoting and sustaining its growth and sulfate-reduction activity.     

Plant species richness does not attenuate responses of soil microbial and nematode communities to a flood event

Human activities are causing climatic changes and alter the composition and biodiversity of ecosystems. Climate change has been and will be increasing the frequency and severity of extreme climate events and natural disasters like floods in many ecosystems. Therefore, it is important to investigate the effects of disturbances on ecosystems and identify potential stabilizing features of ecological communities. In this study, soil microbial and nematode communities were investigated in a grassland biodiversity experiment after a natural flood to investigate if plant diversity is able to attenuate or reinforce the magnitude of effects of the disturbance on soil food webs. In addition to community analyses of soil microorganisms and nematodes, the stability indices proportional resilience, proportional recovery, and proportional resistance were calculated. Generally, soil microbial biomass decreased significantly due to the flood with the strongest reduction in gram-negative bacteria, while gram-positive bacteria were less affected by flooding. Fungal biomass increased significantly three months after the flood compared to few days before the flood, reflecting elevated availability of dead plant biomass in response to the flood. Similar to the soil microbial community, nematode community structure changed considerably due to the flood by favoring colonizers (in the broadest sense r-strategists; c–p 1, 2 nematodes), particularly so at high plant diversity. None of the soil microbial community stability indices and few of the nematode stability indices were significantly affected by plant diversity, indicating limited potential of plant diversity to buffer soil food webs against flooding disturbance. However, plant diversity destabilized colonizer populations, while persister populations (in the broadest sense K-strategists; c–p 4 nematodes) were stabilized, suggesting that plant diversity can stabilize and destabilize populations depending on the ecology of the focal taxa. The present study shows that changes in plant diversity and subsequent alterations in resource availability may significantly modify the compositional shifts of soil food webs in response to disturbances.